In general, the present third generation mobile communication system, especially CDMA TDD (Code Division Multiple Access Time Division Duplex) standard system including the 3.84 Mbps UTRA TDD standard system and the 1.28 Mbps TD-SCDMA standard system, as are known in the art, adopt multi-slot frame structures. The multi-slot frame designed according to UTRA TDD standard has 15 time slots, and the multi-slot frame designed according to TD-SCDMA standard has 7 traffic time slots. A CDMA mode with a maximum spreading factor of 16 is used in every time slot.
FIG. 1 shows a frame structure of the TD-SCDMA. In each 5-ms sub-frame, there are seven traffic time slots: TS0 10, TS1 11, TS2 12, TS3 13, TS4 14, TS5 15, TS6 16, and three special time slots: the downlink pilot time slot DwPTS 17, the main Guard Period G 18 and the uplink pilot time slot UpPTS 19. Every traffic time slot has 864 chips with the duration of 675 μs. Each time slot is divided into four fields: two service data fields 20, 22, one midamble 21 and one guard period 23. The midamble has duration of 144 chips, the GP has duration of 16 chips, and the two data fields are of 352 chips each. When the spreading factor is 16, each of the two service data fields can transmit 22 symbols, which is equivalent to transmission rate of 17.6 kbps in the modulation mode of Quaternary Phase Shift Keying (QPSK).
When a higher transmission rate is needed, multi-channel technique or a lower spreading factor is usually adopted. In this case, the maximum transmission rate of a traffic time slot is 16×17.6 kbps=281.6 kbps. When a much higher transmission rate is needed, multiple slots may be used for transmission, for example, the maximum transmission rate of two traffic time slots can be 563.2 kbps, the maximum transmission rate of three traffic time slots can be 844.8 kbps, and so on.
In summary, at present, all of the third generation mobile communication systems adopt the CDMA technology. In the TDD CDMA standard, in order to support asymmetric DL/UL allocation, multi-slot frame structure is adopted; and in order to provide high-speed data services, variable spreading factor, multi-channel or multi-slot techniques are utilized.
Nevertheless, the foregoing approaches have the following disadvantages:
1. The transmission rate of the data is not high enough. Take the TD-SCDMA system as an example. According to the requirements of the 3GPP specifications, when the transmission rate is less than 8 kbps, only one code channel in one time slot is needed for data transmission; when a voice signal with a transmission rate of 12.2 kbps is to be transmitted, it can be transmitted with two code channels in one time slot or by decreasing the spreading factor to 8. But in either case, the transmitting power of a terminal would be double of the transmitting power needed for a channel with a spreading factor of 16. As mentioned above, the maximum transmission rate of one time slot is 128 kbps, then a service requiring a transmission rate of 384 kbps needs at least 3 time slots. And when the channel coding takes R=⅓, the ratio of puncturing is very high, which results in obviously degraded performance. In order to transmit a service with a transmission rate of 2 Mbps in the TD-SCDMA system, 5 time slots and the 8PSK modulation mode are required. As a result, one carrier frequency can only provide one-way transmission of a service requiring the rate of 2 Mbps. Therefore, up to now, how to increase the data transmission rate of a CDMA TDD system is still an unsolved problem.
2. It is quite difficult to design the user terminal. Regardless of which kind of CDMA TDD technique of the third generation mobile communication standard (UTRA TDD or TD-SCDMA) is adopted, a midamble (as shown by 21 in FIG. 1) is incorporated in a frame, and the joint detection technique is used. Because of the complexity of the joint detection algorithm, the multi-user detection for the user terminal designed at present could be used on the signal of a single time slot, which means the user terminal at present can receive the signal from at most one time slot. In other words, the maximum transmission rate of a service supported by some low-end user terminals is 128 (or 144 ) kbps. To support 384 kbps or 2 Mbps service, the user terminal must have multiple demodulators, which inevitably lead to the user terminal a bigger size, more power consumption, and higher cost.
3. The coverage of the cell is limited. Based on the operating principle of CDMA, when using a lower spreading factor or a plurality of code channels to transmit the traffic data, the transmitting power would be multiple times of the power by using a single channel with a high spreading factor. If the maximum transmitting power of the user terminal is fixed, the communication distance is limited, so is the cell coverage; if the transmitting power of the user terminal is big enough, when the power consumption is increased, the operating time of the batteries of the user terminal is decreased accordingly.